1. Field of the Invention
The invention pertains to the field of monitoring and control of air flow handling devices such as laboratory fume hoods. More particularly, the invention pertains to methods of avoiding false alarms in air flow monitoring systems for air handling systems.
2. Description of Related Art
Laboratory fume hoods have long been used to extract fumes from the vicinity of laboratory workers. Typically, fume hoods provide an enclosure around and above the experimental area or laboratory table, from which an exhaust fan draws air. Room air flows into the hood from vents or an open front to replace the air drawn out by the hood The extracted air is exhausted through appropriate ductwork to the atmosphere in a safe area, typically the roof of the laboratory building. Makeup air flows into the laboratory to replace the room air exhausted by the hoods.
Because of the need to maintain airflow through the hoods for safety reasons, it is important to monitor the hoods and make sure that sufficient air is flowing whenever the laboratory is in use. Failure of a fan motor or plugging of air filters, inlets or outlets, could lead to a dangerous diminution in air flow. Therefore, fume hood airflow alarms are provided for this purpose. These alarms monitor the air flow in the hood or the ductwork leading to or from the hood, whether directly or by monitoring pressure differentials, and warn personnel when the flow drops below a predetermined set point.
Moss, U.S. Pat. No. 4,934,256, “Fume Hood Ventilation Control System”, shows a fume hood equipped with such an airflow monitoring system. U.S. Pat. Nos. 5,439,414 and 5,562,537, assigned to Landis & Gyr Powers, Inc., of Buffalo, N.Y., for “Networked Fume Hood Monitoring System” are examples of networked systems for monitoring a plurality of fume hoods in a facility.
Overall ventilation for laboratory buildings is usually provided by one or more air handling units (otherwise known as Heating, Ventilation and Air Conditioning or HVAC) which draw in outside air, heat or cool it as needed, and distribute the air to the various areas in the building. Air vents and, if needed, exhaust fans, provide an exit route for the conditioned air.
In recent years, concerns for energy conservation have led to buildings being made ever more air-tight and energy efficient. This has resulted in a number of problems in laboratory or factory buildings and other similar facilities in which there are a large number of devices such as lab hoods, paint booths, assembly line process equipment using chemicals, stove hoods, etc., extracting conditioned air from the building. When these devices are on, a significant amount of air is pulled from the interior of the building and exhausted to atmosphere. With the building being made as air-tight as possible, it is no longer feasible to depend on leaks around and through windows and doors to replace the extracted air flow. Outside air must be drawn in through the building's air handling equipment to make up for the air leaving through the hoods. Therefore, it is important that the air handling equipment be running when the hoods are on.
It has become common in recent years for all of the HVAC and other machinery in a facility to be controlled and monitored centrally, with a facility network bus running around the building providing communications for data and commands for all of the equipment. Application Specific Controllers (ASCs) provide interfacing between one or more pieces of equipment and the bus, and a Network Control Module (NCM) connected to the bus monitors and controls the equipment through the ASCs. The NCM may be a stand-alone system or might communicate with one or more conventional microcomputers to provide data monitoring, control and alarm functions using custom or vendor-supplied software, such as the “Metasys” software from Johnson Controls, of Milwaukee, Wis. Johnson Controls also manufactures ASCs and NCMs which are useful with the present invention.
If the hoods are left on when there is no laboratory activity, a great deal of energy is wasted drawing in outside air through the air handlers, conditioning it, and blowing it out through the roof through the hoods. It would seem logical, then, to shut off hoods and other exhaust devices when there is no longer any need for them. This may be done by manual controls on the hoods, but laboratory users can forget to shut down equipment when they leave. Simple time clocks can provide a shut-off function at night, as well.
However, if the building is energy efficient, it is not advisable to simply shut down all of the exhaust equipment, as it is important to keep at least a minimal air flow through the facility to maintain fresh air and keep the temperature within limits. Better than just shutting off the hoods, then, is to reduce the air flow through the devices through variable speed drives on the exhaust fans. Thus, when the laboratory shuts down at night, the hood fans can be automatically slowed down to 25%–50% of normal speed.
A problem arises when this is done. Even though the hoods are not shut off completely, the air flow monitors in the fume hoods will detect the reduction in air flow, and at some point will set off a low air flow alarm. The alarms can be set to a low enough flow that the minimum flow should not set them off, but it is not practical to set this limit too low, or the alarm would not perform its function during the day when it is needed to perform its safety function. Small natural variations in flow can thus cause annoying false alarms as the hood monitors incorrectly interpret a brief drop or rise in hood flow at night as a failure in ventilation. Many facilities have dealt with this by either disconnecting the alarms, to the detriment of facility safety, or keeping the hoods running wastefully 24 hours a day, requiring increased costs of running the HVAC when it is not needed.